CarPlay Navigation Volume Issue

I’m hoping someone can educate me regarding how an iPhone transmits audio via wireless CarPlay, so that I can fully explain (in technical terms) my issue to a head unit manufacturer/supplier.

I’ve recently installed a new head unit, and I’ve found that when playing music and using Google Maps simultaneously in wireless CarPlay mode, the navigation audio volume is not loud enough, even though the music volume is automatically reduced when navigation instructions commence, and I’ve already got “Guidance volume” set to “Louder” in Google Maps.

The head unit does have a “Navigation” volume slider in the settings menu but this has no effect, and the only volume slider which does increase the volume is “PhoneLink”, but that seems to simultaneously increase the volume of the CarPlay music audio stream AND the CarPlay navigation audio stream, which makes the music audio too loud when navigation audio stream stops and music volume returns to normal.

Now, since the head unit is automatically reducing the CarPlay music volume when navigation instructions commence, I assume that it’s able to differentiate (and independently control) the two audio streams automatically, and the obvious conclusion is that the “Navigation” volume slider SHOULD enable the user to manually increase or decrease the volume of the  CarPlay navigation audio stream, and is simply not doing so.

I think I’ve read somewhere that iOS transmits CarPlay audio via multiple streams or channels, and that a receiving device (my head unit, in this case) can therefore control each stream/channel individually.

In this case, I’m assuming that Google Maps navigation prompts are being transmitted on a specific audio stream/channel, and that the head unit’s “Navigation” volume slider should be controlling this stream/channel, but I’m lacking the technical knowledge to communicate this in technical terms to the head unit supplier.

Please can someone confirm if my understanding is correct, and tell me what technical terminology would explain this issue to the head unit supplier.

A link to an online technical explanation would also be useful.

Many thanks in advance.